1. Field of the Invention
This invention relates to a metallic gasket used to seal the joint surfaces of a cylinder head and a cylinder block of a multicylinder internal combustion engine.
2. Description of the Prior Art
A metallic gasket formed out of a metallic material has been used to seal the opposite joint surfaces of structural members, such as a cylinder head and a cylinder block of an internal combustion engine. In a recent internal combustion engine, the increasing of the output level and the decreasing of the weight have been demanded. In order to meet these requirements, a cylinder head and a cylinder block tend to be formed out of an aluminum material having a low specific gravity instead of steel or a cast metal having a high specific gravity which has heretofore been used. An aluminum material has a small weight but the rigidity thereof is low. Therefore, while an internal engine in which the cylinder head and cylinder block are formed out of an aluminum material is operated, the relative displacement of the cylinder head with respect to the cylinder block tends to increase. A metallic gasket has beads near the circumferences of the through holes therein which correspond to the cylinder bores and passages for water and oil. When the cylinder bead and cylinder block are fixed by tightening the same with bolts, these beads form annular resilient contact members with respect to the opposite joint surfaces of the cylinder head and cylinder block and seal these joint surfaces.
There is recently a tendency for a cylinder head and a cylinder block, between which such a metallic gasket is to be inserted, to be formed out of an aluminum material for the purpose of reducing the weight of an internal combustion engine, and the rigidity of these structural members of engine therefore necessarily decreases. When the joint surfaces of these structural members are tightened with locking bolts via a metallic gasket consisting of a single plate, the opposite joint surfaces are liable to be fixed in a non-aligned state since the holes for these locking bolts scatter in the outer peripheral portion, or a portion comparatively close to the outer peripheral portion, of the metallic gasket, i.e., since these holes are not necessarily distributed uniformly around the cylinder bore-aligned holes in the metallic gasket. Consequently, a high-temperature and high-pressure combustion gas enters a narrow clearance between the portions of the opposite joint surfaces of the structural members which are distorted greatly, such as the portions of the same joint surfaces which are among the cylinder bore-aligned holes, to cause the beads on the metallic gasket inserted between these joint surfaces to be corroded and soiled, whereby the sealing effect of the metallic gasket lowers.
During a combustion cycle of an internal combustion engine, the distance between the cylinder head and cylinder block increases and decreases repeatedly, and stress, i.e. mechanical stress and thermal stress are exerted repeatedly on the metallic gasket which is provided between the cylinder head and cylinder block. This stress due to load change is most pronounced in the portions of the cylinder block and cylinder head which have the lowest rigidity. As a result, permanent set in fatigue and cracks occur in the beads to cause the sealing performance of the metallic gasket to be deteriorated.
There is a metallic gasket provided with a layer of a soft metal or a spacer member on a top portion of each bead for the purpose of improving the sealing performance of a high-pressure portion to be sealed, i.e. a portion around each cylinder bore-aligned hole. In both of such metallic gaskets, layers of a soft metal and spacer members are attached to the top portions of the beads to increase the height thereof. When the joint surfaces of the structural members are pressed against each other by tightening the cylinder heal with respect to the cylinder block, the metallic gasket is compressed stronger by as much as an increase in height of each bead due to the addition of the layer of a soft metal or spacer member. Consequently, the improvement of the sealing performance of the portions of joint surfaces of the structural members which are around the cylinder bore-aligned holes can be expected. Since these members of a soft material form padding, it is considered that the padding scatters an extreme pressure, improves the sealing performance of the metallic gasket and joint surfaces or protects the beads.
However, since the portions of the metallic gasket on which the layer of a soft metal or spacer members are provided are limited to the beads, the number of high-pressure portions on the joint surfaces, i.e. the number of seal portions does not basically increase, and aligning the layers of a soft metal or spacer members with the beads and fixing the same thereto during the manufacture of a metallic gasket is difficult and causes an increase in the manufacturing cost.
There is another known metallic gasket (disclosed in, for example, Japanese Patent Laid-Open Nos. 101574/1988 and 149479/1988) other than the above-described metallic gaskets which is provided with beads projecting from the circumferential portions, which form portions to be pressure-sealed, of cylinder bore-aligned holes, as well as grommets attached to the circumferences of the cylinder bore-aligned holes. Even when the grommets attached to the circumferences of the cylinder bore-aligned holes in such a metallic gasket is first exposed to a high-temperature high-pressure fluid, such as a combustion gas, they shut off the fluid, and, even when the fluid should leak from the grommets, the beads shut off the same. These publications also disclose an idea of correcting the irregularity of a clearance between the opposite joint surfaces of the cylinder head and cylinder block by tightening these structural members. However, attaching grommets to the circumferences of the cylinder bore-aligned holes is accompanied by troublesome operations, such as the production of grommets and the fitting and deforming of the grommets around the cylinder bore-aligned holes, and causes the manufacturing cost to increase.
The applicant of the present invention developed a metallic gasket shown in FIGS. 29 and 30, and filed previously Japanese Patent Application No. 306295/1990 therefor. This metallic gasket 20 has beads 24A, 24B formed on the portions of an elastic metallic plate 21 which are spaced from the circumferences of cylinder bore-aligned holes 22A, 22B in the same plate 21 in the radially outward direction thereof, so as to extend along the circumferences of these holes 22A, 22B, and folded portions 25A, 25B formed by folding back the portions of the metallic plate 21 which are around the cylinder bore-aligned holes 22A, 22B in the radially outward direction thereof, the folded portions 25A, 25B of the elastic metallic plate 21 being formed to a predetermined thickness and heat treated. In this metallic gasket 20, a thickness regulating metallic plate 26 is laminated on the portions of the elastic metallic plate 21 which are on the radially outer side of the beads 24A, 24B and on the side of the plate 21 on which the projecting surfaces of the beads 24A, 24B extend, and metallic plates 27A, 27B are held in the folded portions 25A, 25B.
The applicant of the present invention has already proposed another metallic gasket 30 which is shown in FIGS. 31 and 32 and identical with the above proposed metallic gasket having beads and folded portions, except that it also has regulating plate 36 adapted to regulate the strength of the elastic metallic plate and laminated on the surface of the elastic metallic plate on which the projecting surfaces of the beads 34A, 34B extend, in such a manner that the regulating plate 36 extends to the regions which are on the radially outer side of the beads 34A, 34B. In this metallic gasket 30, soft members 37A, 37B are held in the folded portions 35A, 35B (refer to Japanese Patent Application No. 156189/1991).
However, these metallic gaskets have the following technical problems left unsolved. Namely, the metallic gasket 20 has a metallic plate 26 laminated on the side of the elastic metallic plate 21 on which the projecting surfaces of the beads 24A, 24B extend, for the purpose of regulating the thickness of the plate 21 but the portion of the plate 21 on which the metallic plate 26 is laminated is on the radially outer side of the beads and does not include the portions thereof on which the beads are formed. Accordingly, the portions of this metallic gasket on which the beads are formed consist basically of a single elastic metallic plate.
The above-mentioned metallic gasket 30 has a regulating plate 36 laminated on the regions of the elastic metallic plate 31 which correspond to the beads 34A, 34B and the regions of the plate 31 which are on the radially outer side of the beads 34A, 34B, for the purpose of regulating the strength of the elastic metallic plate but the portion of this metallic plate 31 on which this regulating plate 36 is laminated is on the side thereof on which the projecting surfaces of the beads 34A, 34B extend. The region of the plate 31 on which the regulating plate 36 is laminated are the portions thereof which correspond to the beads and the portion thereof which is on the radially outer side of the beads, and this regulating plate 36 is not laminated on the portions of the elastic metallic plate 31 which are on the radially inner side of the beads 34A, 34B.
Therefore, when the metallic gasket 30 is held between the joint surfaces of the cylinder head and cylinder block and tightened, a required difference in height is obtained between the folded portions 35A, 35B and the regions of the elastic metallic plate 31 which is on the outer side of the beads in the radial direction of the cylinder bore-aligned holes 32A, 32B. However, it is very troublesome to carry out a plate thickness control operation in which this difference in height is determined taking into consideration not only the thickness of the elastic metallic plate 31, the height of the beads 34A, 34B and the thickness of the folded portions 35A, 35B but also the thickness of the regulating plate 36, and a required difference in height has to be obtained by only forming the folded portions 35A, 35B. In addition, even if the minimization of a load stress in the beads and the prevention of a full compression of the beads are attained, it cannot be said that this metallic gasket has a sufficiently high durability.